1. Field of the Invention
The present invention generally relates to an antenna and, more particularly, to an integrated feed broadband dual polarized antenna.
2. Background Description
An antenna is a system of wires or other conductors used to transmit or receive radio frequency (RF) or other electromagnetic (EM) waves. An antenna may be formed of one or more lengths of electrical conductors which serve as xe2x80x9cradiating elementsxe2x80x9d, where each different length of the radiating element will resonate at a different frequency. Additional metal or dielectric elements may be added to the radiating elements in order to modify its electrical characteristics.
Antennas and antenna arrays have a wide array of applications. One such application may be in Electronic Support Measure (ESM) systems. In ESM systems, the antennas and other electronics are used to detect emittter signals and estimate several different parameters to provide situational awareness in an electronic warfare environment. These parameters may include, for example, (i) signals characteristics such as RF frequency, pulse width, pulse repetition interval and the like and (ii) incident wave information such as angle of arrival (AOA) (i.e., information concerning the angular direction of a signal source) and polarization for which estimation performance falls into the antenna subsystem design.
ESM systems, however, are polarization sensitive. By way of example, antennas that are circularly polarized can only detect the same sense of circularly polarized emitter signals or any linearly polarized emitter signals. Thus, if emitter signals are opposite sense to these antennas, the system will have poor signal strength and will result in incorrect AOA information. Also, although circularly polarized (CP) antennas can detect any linearly polarized emitters, the axial ratio of the given antenna becomes the key performance factor for AOA accuracy. This is because even a CP antenna responds differently to different linearly polarized signals which, in turn, depends on its circularity or its axial ratio. As a result, AOA accuracy can be compromised due to single calibration used to cover a broad range of linearly polarized emitters, e.g., vertical, horizontal, slant 45xc2x0, etc.
Thus, it is known that current CP antennas provide only one polarization state such as, for example, left hand circular (LHCP) or right hand circular (RHCP) polarization. By way of more specific explanation, a right-hand circularly polarized antenna will detect linearly polarized or right-hand elliptical or circular polarized radiation, but will not detect electromagnetic radiation of the left-hand polarization sense. Therefore, a system equipped with conventional single-sense antennas, such as broadband spiral antennas, will be unable to detect electromagnetic radiation of the opposite polarization sense (i.e., both LHCP and RHCP are not available for system processing), and would thus be xe2x80x9cblindxe2x80x9d to some threats.
A dual polarized antenna is thus needed to:
1. Cover a broader range of emitter polarizations;
2. Estimate emitter polarization and thereby obtain better emitter identification; and
3. Improve AOA accuracy through calibration over two orthogonal polarizations which are sufficient to span all polarization states.
To attempt to overcome some of the problems of known antennas, several different designs have been presented. For example, dual polarized microstrip elements have been known for years, but these systems have a very narrow band and are not applicable in ESM systems. Also, qual-ridged horns are known which are based on dual probe feeds inside waveguide structures. These designs provide dual polarization functions, but are bulky and costly because both the element and feed are not linear and cannot be manufactured with low cost printed circuit technology. Another known system is sinuous antennas (see, for example, U.S. Pat. No. 4,658,262) which have a planar design manufactured with printed circuit processing. This type of system, however, requires complex fanded network or multiple baluns that run along the perpendicular axis. These feed networks and baluns are not easily realized with printed circuit technology which, in turn, increases the costs. These types of systems additionally are prone to vibration breakdown, which is not very advantageous in aviation applications.
To further attempt to solve the problems of known antenna systems, for example, U.S. Pat. Nos. 6,211,839 and 5,164,738, both include a planar structure. However, in both of these systems, the radiating arms are interleaved which provide poor polarization purity. Thus, when one radiating arm is excited, an adjacent, interleaved radiating arm will also become excited due to leakage. This contributes to cross polarization. These systems further include, in embodiments, curved surfaces as well as curved radiating arms, both of which provide for polarization wobbles along the curved arms thereby making calibrations unstable over operating frequency range.
In a first aspect of the present invention, an integrated feed broadband dual polarized antenna is provided. The antenna includes a substantially planar support structure having a first surface and a second surface. A plurality of symmetrically positioned radiating structures are disposed on the first surface of the planar support structure and a plurality of straight, non-interleaving ribs extend substantially perpendicularly from a first and second lengthwise side of each radiating structure. The plurality of straight, non-interleaving ribs extending from the first lengthwise side are complimentary to the plurality of straight, non-interleaving ribs extending from the second lengthwise side of the each radiating structure. Integrated microstrip lines serve as transmission lines to feed and are disposed on the second surface of the planar support surface and run along a length and are connected to two orthogonal, adjoining radiating structures of the plurality of symmetrically positioned radiating structures. The integrated microstrip lines form an integrated printed circuit infinite balun structure which is inherently frequency independent and provides 180xc2x0 electrical phase required to feed a 180xc2x0 rotated radiating structure.
In embodiments, the plurality of straight, non-interleaving ribs eliminates coupling or interference between respective, adjacent straight, non-interleaving ribs thereby providing pure polarization. Additionally, each of the plurality of straight, non-interleaving ribs have an angled connecting end to lengthen each of the straight, non-interleaving ribs. The length at resonance must be naturally quarter wave; however, the distance where the rib is located from the center point can control he H-plane beamwidth. The plurality of straight, non-interleaving ribs are also scalable and are isolated from ribs which extend from an adjacently positioned radiating structure. This eliminates interference between the non-interleaving ribs. The straight rib design provides a longest resonant length within a square aperture and thus maximizes available space for operating in a low frequency range. Additionally, the average cross coupling between the ribs is on the order of approximately xe2x88x9225dB.
In another aspect of the present invention, the integrated feed broadband dual polarized antenna includes a substantially planar support structure and a plurality of symmetrically positioned structures radiating at 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0 from a central point on the first surface of the planar support structure. A plurality of complimentary, non-interleaving ribs extend perpendicularly from a first and second lengthwise side of each radiating structure, and integrated microstrip lines form an integrated printed circuit infinite balun structure. The plurality of complimentary, non-interleaving ribs eliminates coupling or interference between respective, adjacent non-interleaving ribs.